The present invention relates to a method for calibrating equipment for determining the surface uniformity of film or sheet material, said equipment comprising a measuring position in which the material is arranged to pass over a rotating measuring shaft, a light source arranged to illuminate the horizon formed by the outer surface of the material exactly where it is in contact with the periphery of the measuring shaft, and a light detector for detecting light transmitted from the light source and emitting corresponding electric signals to subsequent signal-processing apparatus in order to determine the dimensions, number and locations of the irregularities from the shadows produced by the surface irregularities.
When an impurity appears in material such as polyethylene tape or film, it migrates for hydrodynamic reasons towards the surface of the tape or film and is deposited in the surface so that approximately half its diameter protrudes from the surface. Surface uniformity determination is therefore included when checking the quality of such tape or film.
U.S. Pat. No. 4,656,663 describes a system for inspecting transparent film, primarily intended for use as videotape, to detect irregularities in the surface of the film. A sample of the film is coated with metal by means of steaming on, the metal being applied at a certain angle. The film is then illuminated and the distribution of irregularities is determined with a scanning video camera from the number and length of the xe2x80x9cshadowsxe2x80x9d obtained in the metallization, caused by irregularities in the surface of the film. The signal obtained at the scanning is digitalized, stored in a computer memory and processed pixel by pixel.
The present invention relates to equipment for determining the surface uniformity of film or sheet material which is caused to pass over a measuring shaft and the horizon formed by the outer surface of the material where it is in contact with the periphery of the measuring shaft is illuminated and the light transmitted is detected. The dimensions, number and locations of the irregularities are detected from the transmitted light detected, which is related to shadows produced by the surface irregularities. The object of the invention is more specifically to achieve a new, reliable method and provide a new, reliable device for calibrating this type of equipment for determining surface uniformity.
This object is achieved with a method and a device of the type described in the introduction, having the characteristics defined in claim 1 and claim 8, respectively.
A very substantial advantage with the calibration method and device according to the invention is that the calibration can be traced backwards, which is a requirement according to the ISO 9000 standard.
The equipment for determining surface uniformity is intended for use for film or sheet material, e.g. tape of various thicknesses, that is to say material extruded at various speeds to various thicknesses. According to an advantageous embodiment of the method according to the invention calibration shafts are produced having different diameters corresponding to different material thicknesses. The equipment is thus calibrated for different material thicknesses, using different calibration shafts.
According to another advantageous embodiment of the method according to the invention said elevations are produced on the calibration shafts by applying drops of glue on the peripheral surfaces of the shafts and causing them to harden. Before the glue, suitably epoxy plastic, hardens the drops spread out so that their shape substantially resembles a normal distribution curve. Hardening is suitably performed in heat.
According to yet another embodiment of the method according to the invention the actual dimensions of the elevations are determined with the aid of a reading microscope. The relative uncertainty of this determination is estimated to 1% from the spread in measured data and also estimated uncertainties in the equipment used.
According to another embodiment of the method according to the invention the calibration shafts are rotated several turns to enable determination of hit rate and standard deviations, as well as determining the height and width of the elevations.
According to yet another advantageous embodiment of the method according to the invention a spectrography camera is used as light detector, said camera emitting signals representing light transmission data scanned pixel by pixel.
According to a further advantageous embodiment of the method according to the invention, before measurement is commenced on the calibration shaft a scale is applied in the measuring position of the detecting equipment, with the aid of which the desired pixel resolution is adjusted and the camera focus fixed on the measuring position. The scale used is suitably a commercially available Heidenhain scale designed for such purposes. The camera is fixed mechanically following correct setting and thereafter maintains this correct setting.
To explain the invention further a number of examples of the device according to the invention will be described in more detail by way of example, with reference to the accompanying drawings in which